What would happen, he wondered, if he laid a street grid of today's city over this 18th-century rendering? Would anything line up? To find out, Sanderson enlisted family and friends, starting with his wife, Han-Yu Hung, and their young son, Everett, to join him on weekend expeditions to visit places on the map that still existed. Trinity Church in Lower Manhat­tan, for one, was founded in the late 17th century. A typical grave marker in the church cemetery reads, "Here Lyeth the Body of John Abrell Who Departed this Life Jan the 10th 1762 Aged 40 Years." Since the cemetery can be located on both the "British Headquarters Map" and today's street grid, Sanderson was able to push a virtual pin, so to speak, through both maps by taking a GPS reading at the site and attaching it to a digitized version of the older map. After repeating this process at 200 or so places, sticking in pin after pin, he and his team succeeded in matching the "British Head­quarters Map" to today's city grid with an accuracy of half an uptown block, or roughly 130 feet. For Sanderson this added a whole new dimension to the modern city's landscape. He could now stand at any spot in Manhattan and picture, more or less, what had been there in 1782.

Take the gentle rise of Fifth Avenue as you walk past the New York Public Library. "There's a reason you can stand on the sidewalk here and see the tops of people's heads a few blocks away," Sanderson said. "This was near the top of Murray Hill, where the Murray family had a farm and orchard in 1782. During the battle for New York, the British landed at Kips Bay on the East River and marched up here, cutting off half of Washington's army, which was trapped in Lower Manhattan. There's a legend that Mrs. Murray offered tea to the British officers. So they stopped here at the farm, and while they were having tea, Washington's troops slipped past them on the Bloomingdale Road, which is now Broadway, and escaped."

As fascinating as the "British Headquarters Map" was, Sanderson didn't want to stop his time machine at 1782. He wanted to go all the way back to 1609. So he and his colleagues stripped from the map all the features that had been added by settlers and soldiers—such as roads, farms, and fortifications—until they'd reduced their digitized version of the map to the basic building blocks of the physical landscape: shorelines, hills, cliffs, land cover, streams, and ponds. As a landscape ecologist, Sanderson was used to taking apart wild places conceptually to understand how they work, separating a rain forest in Gabon, say, into geological, hydrological, ecological, and cultural layers. Now he and his colleagues set out to build a landscape from the bottom up, starting with the terrain and filling it with all the plants and animals that were likely to have lived there.